The invention relates to electron gun assemblies and more particularly to in-line electron gun assemblies of the type used in color television picture tubes.
U.S. Pat. No. 4,063,128 issued to R. H. Hughes on Dec. 13, 1977 discloses a structure for maintaining the equality of expansion in time and magnitude of the cathode-to-control grid (also referred to as the G1 grid) spacing in an in-line electron gun assembly in order to provide simultaneous cutoff of the electron beam currents for black level adjustment. In the Hughes patent, assigned to the same assignee as the present invention and incorporated herein for disclosure purposes, the cathode structure comprises three separate cathode assemblies disposed in a plane. Each of the cathode assemblies comprises a cathode sleeve closed at one end with an electron emissive coating disposed on the closed end portion of the sleeve. A filament is mounted within each of the sleeves. The sleeves are attached to cathode eyelets which are affixed to beading support members embedded into a pair of oppositely-disposed glass support rods. The outer support members are formed of 0.51 mm thick material to provide structural rigidity while the center support member is formed of 0.25 mm thick material to permit adequate spacing between the center and outer cathode assemblies. The thicker outer support members provide a better path for conducting heat away from the cathode filaments than does the thinner center support member. Consequently, when thermal equilibrium is achieved at approximately 15 minutes after filament activation, the center cathode assembly is operating at a higher temperature than the outer cathode assemblies. In other words, the temperature rise during warmup is greater for the center cathode assembly than for the outer cathode assemblies. To compensate for the temperature differences among the cathode assemblies, Hughes makes the outer cathode eyelets of a material having a higher thermal expansion coefficient than the material used to make the center cathode eyelet. This permits the outer eyelets to expand at substantially the same rate as the center cathode eyelet, thereby maintaining the change in cathode to G1 grid spacing substantially equal from gun to gun. In the Hughes patent, the center eyelet is constructed of type 52 metal while the outer eyelets are formed of type 305 stainless steel. The material used for all the beading support members in the Hughes structure is type 305 stainless steel.
While the structure disclosed in the Hughes patent provides satisfactory simultaneous cutoff of the beam currents for black level adjustment, the color tracking of the outer beams, i.e., "blue-red tracking", is occasionally a problem. While the cathode sleeves and cathode eyelets expand along the electron beam axis, i.e., along the Z-axis in a manner predicted by the Hughes patent, the color tracking problem appears to be related, to a significant extent, to nonuniform stressing of the cathode support members along the X-Y axis during warmup.
U.S. patent application Ser. No. 326,348, filed on Dec. 1, 1981 by M. K. Brown et al. and assigned to the same assignee as the present invention, discloses a cathode support assembly in which the stresses of the beading support members are equalized along the X-Y axis. More specifically, the two outer support members are formed from type 52 metal having a thermal expansion coefficient between about 9.6 to about 10.1 microns per meter per degree centrigrade and the center support member is formed from type 42 metal having a thermal expansion coefficient between about 4.0 to about 4.7 microns per meter per degree centigrade. In this structure, the outer beading support members expand more than the center beading support member but substantially equal to one another so that the cathode assembly is rigidly affixed to the glass support rods.
While the Hughes structure and the Brown et al. structure generally improves the performance of the in-line electron gun, the relatively low expansion support members of the cathode assemblies of the Brown et al. structure introduce a thermal stress problem into the electron gun assemblies that affects the control grid electrode G1. FIG. 3 shows a graph of cathode current versus minutes warmup for the three electron guns of the Hughes structure. While the cathode currents for the three electron guns vary somewhat during the first several minutes of warmup, the cathode currents are not excessive and the major drawback of the Hughes structure, in addition to the aforementioned "blue-red tracking" problem, is that it takes about 7.5 minutes of warmup for the cathode currents to differ by about 2 miroamperes or less. In the Brown et al. structure, the cathode-to-control grid spacing is greatly influenced by the different materials of the support members. It has been determined that in the Brown et al. structure, the plane of the control grid, G1, moves away from the cathodes as the electron gun elements approach thermal equilibrium. Accordingly, to achieve the desired cathode-to-control grid spacing at operating temperature, the room temperature cathode-to-control grid spacing must be reduced. This reduction in cathode-to-control grid spacing at room temperature in the Brown et al. Structure results, as shown in FIG. 4, in excessive cathode current for all three electron guns for about the initial 4.5 minutes of operation after which the cathode currents reach an acceptable value of less than about 12 microamperes of current and the variation between the three electron guns is about 2 microamperes or less.